Deposited metal films formed by chemical vapor deposition (CVD) methods are common practice for a variety of semiconductor and micro-electronic applications. Attempting to deposit metallic thin films for coatings using CVD methods has proven to be difficult. Difficulties may be particularly pronounced when producing thick films (1 to 2 microns thick or greater), such as protective coatings including corrosion resistant coatings.
Several metal deposition approaches have been published that utilize solid, liquid and gaseous metal precursors that are volatilized and introduced into a reaction chamber, where the precursor is decomposed into its constituent components with the metal moiety being deposited onto a target substrate. Although metallization of a substrate has been achieved, the poor quality of thin film formation has prohibited economically attractive applications of such coatings.
Aluminum metal is a logical choice for thin-film coatings for the corrosion protection of a substrate. Typical methods of applying aluminum coatings have been physical vapor deposition (PVD) methods including ion vapor deposition (IVD). These methods have provided aluminum coatings, but require expensive deposition equipment and are difficult to operate and maintain.
Other methods, such as CVD, have provided aluminum coatings but have been difficult to control with the decomposition of the aluminum precursor throughout the deposition chamber causing aluminum dust in the transport medium and subsequently on the surface of the target substrate. The formation of dust particles proves detrimental to the quality of the thin film on the substrate in terms of corrosion protection, surface morphology, and aesthetics.
A liquid phase plating method where the deposition of aluminum is achieved by convection heating of target metals with the subsequent metal plating employing the use of liquid metal alkyl precursors is described in U.S. Pat. Nos. 3,449,144; 3,449,150; 3,464,844; 3,578,494; and 3,707,136. These approaches have issues with temperature control, deposition layer thickness control, and process design equipment requirements.
U.S. Pat. No. 2,700,365 to P. Pawlyk teaches a gas phase method, rather than one utilizing a liquid-containing vapor, for plating surfaces by using carbonyls or other volatile metal-bearing compounds.
J. C. Withers was granted U.S. Pat. No. 3,702,780 which teaches the use of an atomization spray comprising a metal-containing precursor to coat an inductively heated substrate with a plating film. This patent describes monitoring the temperature of the substrate. This technology, termed the “Pyrolytic Spray Technique”, is further described in Chem. Vapor Deposition, Int. Conf., 2nd 1970, 393-407.
Previous attempts to transfer aluminum coatings from CVD laboratory apparatus to larger scale equipment for mass production have proven difficult. Coating integrity has been difficult to attain in larger CVD reactors.